Powder coating, in particular with epoxy resins, is a well known technique. Simply, a pulverulent mixture containing resin, curing agent and other components is applied to a substrate, resin and curing agent melt by heating to temperatures usually higher than 150.degree. C., flow out to form a coherent layer, and react to harden the coating. Curing agents for this application should be latent, that is non-reactive at room temperature, and fully reactive above about 150.degree. C. For many uses amine-type curing agents, for example, based on dicyandiamide, have given excellent results; however, such coatings have a tendency to yellowing when stored for a prolonged time at temperatures of 180.degree.-200.degree. C.
British Pat. No. 1,399,377 discloses the use of certain phenolic compounds as cross-linking or curing agents in epoxy resin powder compositions; the phenolic compounds are simple dihydric phenols such as (Bisphenol A), 2,2-bis(4-hydroxyphenyl)propane, or resinous adducts prepared by reacting a diglycidylether of Bisphenol A with an excess of dihydric phenol at 150.degree. C., optionally in the presence of a catalyst, for such a time that the cooled product is pulverizable and suitable for use as a cross-linking agent. The phenolic compounds are then mixed with the solid epoxy resins in a ratio of from about 0.8 to 1.1 phenolic hydroxyl groups per epoxy group; the powder coating compositions may also contain other usual additives for thermosetting coating compositions, such as curing catalysts, pigments, flow control agents, antistatic agents, etc. The epoxy resin components of the powder were copolymers of glycidyl methacrylate, and calculation shows that the copolymers as described contain in general more than two epoxy groups per molecule.
Phenolic adducts of this type and their use as curing agents for epoxy resins have also been disclosed in U.S. Pat. No. 3,931,109, wherein it is recommended to use the phenolic adducts from dihydric phenols and diepoxides in combination with epoxy resins which have an epoxy functionality greater than 2.
However, the majority of epoxy resins, in particular for powder coatings, are diglycidyl ethers of dihydric phenols, and use of a dihydric curing agent may require excessive cure schedules to effect sufficient cross-linking and solvent resistence.
Novel phenolic compounds have now been found in which this drawback has been avoided, wherein the residue of a dicarboxylic acid is incorporated in the structure of the phenolic adduct.